U.S. Pat. No. 5,157,260 shows a quadrupole mass filter coupled to an atmospheric pressure ion source by an ion transmission arrangement including a capillary, a conical skimmer and ion optics. A tube lens cooperates with the end of the capillary to force the ions into the center of the ion jet which travels through the conical skimmer. A quadrupole mass filter analyzes the transmitted ion beam to provide a mass spectrum.
U.S. Pat. No. 4,542,293 describes a capillary made of an electrical insulator for conducting ions out of the ionizing electrospray region at atmospheric pressure to a lower pressure region. A conductive coating is formed on the ends of the capillary and a voltage is applied thereacross to accelerate the ions. A skimmer is disposed adjacent the end of the capillary and is maintained at a voltage which causes further acceleration of the ions through the skimmer and into a lower pressure region which includes focusing lenses and analyzing apparatus.
In these and other prior art mass analysis systems, the orifice of the capillary passage which connects the atmospheric pressure chamber to a lower pressure chamber is in line with the outlet of the ion spray device which forms the sample ions for analysis. This arrangement provides excellent performance for the majority of solvent systems and flow regimes used in atmospheric pressure ion (API) analysis. However, when non-volatile buffer systems are used, there is the possibility of fouling of the capillary intake or sampling orifice by deposition of salts from undesolvated droplets that strike the sampling orifice and evaporate. The deposited salts gradually block the flow of sample ions and reduce performance of the overall system by progressively reducing the number of ions which are transmitted to the mass analyzer.